Differentiated link back-up mechanisms are generally provided in the transport network. These mechanisms differ in their restore times in the event of a fault and in the bandwidth consumed to back up the link of the transport network. The faster these mechanisms restore traffic after a fault, the more resources they consume. The following back-up mechanisms can be mentioned by way of example:                1+1 Protection, where the traffic is transmitted simultaneously on two links in parallel;        N:1 Protection, where N links share one back-up link and route their traffic on that link in the event of a fault; and        Restore Protection, where in the event of a fault the network attempts to set up a new path as quickly as possible.        
All of the traffic between two routers is conveyed by one link or by a plurality of links, depending on the transport network back-up level.
This traffic is divided into a plurality of streams having different respective back-up requirements. A stream is characterized by various characteristics common to a plurality of packets. These characteristics, also known as identification elements, can be present in different layers of the OSI (Open System Interconnection) model. They can correspond to the contents of the source and/or destination address fields (layer 3) or to any other field in the headers of the packets, notably the protocol type (layer 3) and the port numbers for TCP or UDP segments (layer 4). Some streams require a very high levels of back-up, for example streams linked to real-time applications requiring breaks to have a duration of less than around 100 milliseconds, while other streams are more tolerant of breaks and therefore do not require a high back-up level. At present, all traffic is backed up to the same level and if a high level of back-up is chosen in the transport network this implies an unjustified consumption of resources to back up streams that would otherwise have required only a lower level of back-up.
In the document entitled “Recovery Time Analysis of Differentiated Resilience in MPLS” by A. Autenrieth, published in the proceedings of the DRCN2003 conference, a method of backing up streams, notably in a label-switched or MPLS network, is defined as a function of a parameter representative of the recovery time, called the resilience class (RC). To each value of the parameter RC there corresponds a back-up level and therefore a back-up procedure in the MPLS network adapted to the required back-up level. For example, for streams to be routed in the MPLS network requiring a very high back-up level, denoted RC1, i.e. a recovery time less than around 100 ms, a back-up label-switched path is set up beforehand on which the streams corresponding to the parameter value RC1 are conveyed in the event of fault in the main path. Accordingly, in the event of a fault, the recovery time of these streams is less than the required value.
In the document “Optical Layer Survivability: a Services Perspective” by O. Gerstel and R. Ramaswani, published in the journal “IEEE Communications Magazine” of March 2000, the authors touch on the problem of networks with architectures conforming to a multiplayer model and identify the requirement for coordination in the event of a fault between the back-up mechanisms between the packet communications network as client of the transport network and the transport network itself. To this end, it defines five service classes for specifying which layer backs up the stream in order to avoid inefficient interaction between the back-up mechanisms. However, the transport network is not informed of the back-up level required by the packet communications network.
There is therefore a need for a technique for backing up traffic in a transport network in a differentiated manner as a function of requirements defined for each stream in a packet communications network in an architecture conforming to a multilayer model.